pylint #19802 false positive: Unused argument/variable on augmented assigment [open]

Maarthen reported

def exclude(setA, setB):
      setA -= setB

W0613:  1:exclude: Unused argument 'setA'

Or a variant with a local variable instead of an argument:

def exclude(func, setB):
      setA = func()
      setA -= setB

W0612:  2:exclude: Unused variable 'setA'

It seems that the left side of "-=" is not considered a use of the argument/variable.

The same message is issued for integers as for sets, but for sets a function like that can actually be useful while for integers it is not. So one could argue the message make sense for integers and other immutable types where the value is replaced rather than modified (such as "+=" for strings).

def subtract(numB):
      numA = 33
      numA -= numB

W0612:  2:subtract: Unused variable 'numA'
prioritynormal
typebug
done in<not specified>
closed by<not specified>

Comments

  • 2010/01/21 13:45, written by sthenault

    skip:

       Maarten> The "setA" variable itself is indeed unused. But the value of
   Maarten> the object that "setA" refers to is most likely used somewhere
   Maarten> outside this function.  For example:

I'd still argue that the warning will catch errors and is still useful.
pylint can't tell whether setA is immutable or not, so it can't tell if the
-= operator has side effects.

   Maarten> a = set(range(10))
   Maarten> exclude(a, set(range(5, 15)))
   Maarten> print a

What if I called it like this?

   exclude(4, 1)

?  That's a perfectly legal, though somewhat silly, way to call exclude().
In fact, when you originally presented just the exclude function without an
example, I saw the -= operator and immediately thought "ah, numbers".
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Ticket #19802 - latest update on 2010/01/21, created on 2010/01/21 by Sylvain Thenault